Q: How to implement sensorless BLDC motor control using back-EMF zero-crossing detection?
Answer
Sensorless BLDC control eliminates the Hall sensor reducing cost and wiring. Back-EMF (BEMF) zero-crossing detection: as the rotor spins the spinning magnets induce voltage in the un-energized stator winding. The zero-crossing point (when BEMF equals the virtual neutral point) occurs at exactly 30 degrees before each commutation. Implementation: (1) Use comparator to detect zero-crossing of BEMF vs neutral voltage. (2) OR use ADC to sample BEMF and detect zero-crossing in software. (3) Apply commutation after a fixed 30-degree delay. Speed is estimated from the period between zero-crossings. Startup: use open-loop ramp (pre-commutate) at low speed until BEMF is detectable (typically above 5-10% of rated speed). Below that use IC-based sensorless startup. Limit: sensorless FOC (SMO/PLL observer) provides better low-speed torque than simple BEMF detection.
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